Understanding Humidity and Dew Point

As with so many of the posts here at The Weekly Pellet, I like to try and demystify things. One topic that seems to cause a lot of confusion, unless you are a meteorologist, is humidity and relative humidity and dew points.

Let’s spend a little time talking about these terms.

We all understand the concept of humidity, or at least we understand that we feel miserable on hot and humid days in the summer and everyone jokes about the summers in Phoenix when it is 117 °F but “hey, it’s a dry heat”. I have heard people say that, even though it was a cold winter day, it did not feel that bad because it was humid that day (impossible). I have also heard people say that if the humidity reaches 100%, that means it’s raining out (not true).

When a meteorologist says that it is 90% humidity out, they are talking about relative humidity or RH. This is not an absolute measurement of humidity. Humidity is water vapor that is dissolved in air similar to salt dissolved in water. Like salt water, warm air will hold more water than cooler air. Relative humidity is the percentage of moisture in the air compared to the maximum amount of moisture that the air could hold at the current temperature of the air. In other words, the amount of moisture relative to the maximum. The maximum varies wildly depending on the temperature.

For example, at 86 °F (30 °C) air can hold about 28 g of water vapor per kg of air. So, 50% Relative humidity would be 14 g water/kg air. At 32 °F (0 °C) air can hold only about 4.5 g water/kg air. At this lower temperature, 50% RH would be 2.25 g water/kg air. As you can see from this example there is a big difference between 50% RH at 32 °F and 50% RH at 86 °F.

A rule of thumb is that the maximum amount of water vapor that can be dissolved in water doubles for every 18 °F (10 °C) increase in temperature. This is why even a high humidity cold winter day is still a very dry day compared to the summer time.

You’re probably wondering how this ties into plastic materials. Well, the reason that understanding this is important is because so many of the materials that we use absorb moisture out of the air and have to be dried before they can be run. In the material supply business, we always know that complaints about nylon will increase during the summer months because drying it properly becomes more of a challenge because the absolute humidity is so much higher than in the winter.

This brings me to the next term which is dew point. Dew point is also a measure of moisture in the air. Plastic material dryers often have dew point meters on them. The reading from the dew point meter is often a source of confusion.

Dew point is the temperature at which a given sample of air would have to be cooled to before it would be saturated with moisture. Cooling the air below the dew point would cause water vapor to condense and form dew. I know this is a bit confusing.

If a sample of air has a lot of moisture in it, cooling it to say 73 °F (23 °C) would cause moisture to condense because at this temperature the air cannot hold as much moisture as it contained in it at say 86 °F (30 °C). If the air was very dry, you would be able to cool it to a very low temperature, maybe below zero, before the water vapor would condense. In this way, knowing the dew point temperature gives you an idea of how much moisture is in the air absolutely instead of relative to the max at a given temperature as with the relative humidity measurement.

Ideally, we could have a sensor that would just tell us how much moisture was in the air absolutely. It would read in grams of water/kg of air. However, this is not that easy to measure. Dew point, however, can be measured relatively easily. Essentially the dew point measurement while not very useful by itself is an excellent indicator of the amount of moisture that the air contains regardless of the current temperature. It is important to know this when drying plastic.

It is also important to understand that the dew point meter on a dryer is not measuring how dry the material is, it is indicating how dry the air in the dryer is. It really just indicates if the dryer is working properly. The only way to tell how much moisture is contained in the material is to use a moisture analyzer.

In a way, the dew point meter is an indicator of an indicator of what you want to know. This is why it is always a good idea to have a moisture analyzer. By itself, the dew point reading on the dryer does not mean anything. All you really need to know is that the dew point meter should be reading between -20 °F and -50 °F (-29 °C and -46 °C). Readings in that range indicate that the dryer is drying the air enough so that it can effectively pull the moisture out of the plastic that is in the dryer.

If you fell asleep reading this, here is what you need to remember:

At a given relative humidity, the absolute humidity doubles for every 18 °F change in temperature.

90% RH in the summer is a lot more humid than 90% RH in the winter.

A dew point meter is an indicator of the amount of moisture contained in the air that is being measured and that a well-functioning dryer will produce a dew point reading of between -20 and -50 °F.